1. Field of the Invention
The present invention relates generally to disk drives such as a hard disk drive, particularly to a gain control of an AGC amplifier included in a read channel.
2. Description of the Related Art
In recent years, in the field of disk drives represented by hard disk drives, development of a longitudinal magnetic recording method and a perpendicular magnetic recording method in which a recording density can be raised has been promoted.
In the disk drive of the perpendicular magnetic recording method, a read signal read from a disk medium (hereinafter referred to simply as the disk) by a read head forms a rectangular signal waveform whose amplitude corresponds to the direction of magnetization. When the read signal waveform is differentiated by a differentiation circuit, the read signal (i.e., the differentiated waveform) is obtained similarly as the longitudinal magnetic recording method.
Therefore, in the disk drive of the perpendicular magnetic recording method, when the differentiation circuit is disposed in a read channel as a signal processing circuit, a data decoder for decoding user data employed in the longitudinal magnetic recording method, or a servo decoder for decoding servo data can be used. Additionally, an actual signal processing circuit is realized as a read/write channel including the read channel and write channel for recording/processing the data and as a single chip LSI circuit.
Additionally, in the disk drive, the disk is always rotated at a constant speed by a spindle motor. Therefore, tracks on the disk have different linear speeds (relative speed of the disk and head) in accordance with positions in a radial direction. Therefore, when the data is recorded with a signal having the same frequency, a linear recording density (bit number of the user data recorded per constant length of a track longitudinal direction) differs with the track on an outer peripheral side and track on an inner peripheral side on the disk. That is, the linear recording density of the track on the outer peripheral side on the disk is reduced.
In order to secure a data storage capacity as large as possible in the disk drive, a recording method called a constant density recording (CDR) method is used in which the linear recording density becomes constant in each track without depending on the position in the radial direction of the disk. Additionally, the linear recording density is set to be constant by each track unit in an ideal CDR method, but a zone bit recording (ZBR) method is actually and practically used.
In the ZBR method, a group of tracks on the disk is formed by a unit called a zone, and a recording frequency of the data (similarly as a reproduction frequency) is set to be the same in the respective tracks included in one zone. In fact, a large number of track groups on one surface of the disk are formed and managed by about 10 to 20 zones.
In the ZBR method, the recording frequency of the data increases in the tracks included in the zone on the outer peripheral side on the disk, but the linear recording density substantially becomes constant as a whole. On the other hand, the recording frequency of each track is constant within the zone, but the recording frequency differs in the different zones. The data is recorded with a high recording frequency in the tracks included in the zone on the outer peripheral side. In a concrete example, in a disk drive having a disk diameter, for example, of 2.5 inches, the linear speed in the outermost peripheral track on the disk is substantially twice the linear speed of the innermost peripheral track. Therefore, in order to keep the linear recording density constant, it is necessary to record the data in the outermost peripheral track at a recording frequency twice that of the innermost peripheral track. That is, if there is a difference of n times in the linear speed between the outer and inner peripheral tracks, the data is then recorded at a recording frequency which is n times the recording frequency, and thereby the linear recording density is held constant.
On the other hand, a transition width of isolated magnetization formed on the disk does not depend on the linear speed, and is formed in a constant length (distance) with respect to a certain combination of the head and disk. Therefore, a transition time width of isolated magnetization is reduced in the tracks included in the outer peripheral zone whose linear speed is high in proportion to the position in the radial direction on the disk.
In general, a magnetoresistive (MR) element or a giant MR (GMR) element is used as a read head in the disk drive. The amplitude of the read signal read by the read head is substantially constant without depending on the position (linear speed) of the track in the radial direction. The read signal corresponds to the data recorded on the disk with the same linear recording density.
Therefore, particularly in the disk drive of the longitudinal magnetic recording method, an average value of the amplitudes of the read signals is substantially the same even from any inner/outer peripheral track. Therefore, an auto gain control (AGC) amplifier for use in the read/write channel (including the signal processing circuit of the read signal) of the disk drive may have only one gain value (hereinafter referred to as the initial gain value) set at an initial time. Additionally, since there is a dispersion in characteristics of the head or the disk, the initial gain value optimized for each disk drive is set. Moreover, the AGC amplifier is an amplifier for controlling the read signal read by the read head so that the amplitude of the signal becomes constant.
On the other hand, in the disk drive of the perpendicular magnetic recording method in which the CDR method or the ZBR method is employed, as described above, the differentiation circuit is disposed in the read/write channel. The differentiation circuit is a circuit for differentiating the read signal in the read channel to reproduce the data from the read signal. The differentiated signal has a signal amplitude which changes in proportion to the position (i.e., the linear speed) in the radial direction of the track. That is, the amplitude value of the read signal (differentiated signal) changes in proportion to the recording frequency for each track or each zone during a read operation. Therefore, there is the following problem.
That is, during the read operation, an AGC acquisition time in the initial operation of the AGC amplifier lengthens. As a result, a read error is easily generated in a data decode processing. When the read head is positioned in the track as an access object, the AGC amplifier included in the read/write channel executes an AGC acquisition processing in the read operation from a first data sector of the track.
That is, the gain control of the AGC amplifier is executed until the amplitude of the read signal from the first data sector reaches a predetermined amplitude value. In the gain control, the initial gain value set for each disk drive is used. For example, when the initial gain value is set to be optimum in the track of an intermediate periphery on the disk, the amplitude of the read signal of the outermost/innermost peripheral track is different from that of the intermediate peripheral track, and therefore the AGC acquisition time lengthens. Therefore, since the read signal having the amplitude thereof insufficiently controlled is subjected to the data decode processing, a read error is easily generated.